1. Field of the Invention
The present invention relates to a method for forming a gate stack which minimizes or eliminates damage to the gate dielectric layer and/or the silicon substrate during gate stack formation. More particularly, the present invention relates to reducing temperature during the fabrication of the gate stack to eliminate the formation of silicon clusters within the metallic silicide film of the gate stack. The present invention also includes methods for dispersing silicon clusters prior to the gate etch step.
2. State of the Art
The operating speed of semiconductor devices in very large scale integration (xe2x80x9cVSLIxe2x80x9d) and ultra large scale integration (xe2x80x9cUSLIxe2x80x9d) depends primarily on the resistivity of the conductive material (hereinafter xe2x80x9ctrace materialxe2x80x9d) used to transmit signals from one circuit component to another circuit component. Additionally, in order to increase the circuit component density and/or reduce the complexity of the metal connections between the circuit components, a highly conductive trace material layer is required on the gate stack. Thus, the trace material must be a low-resistivity material.
Metallic silicides have recently become popular for use as low-resistivity trace material. Tungsten silicide (xe2x80x9cWSixxe2x80x9d) has become a leading low-resistivity trace material. Various etching chemistries have been developed to pattern the WSix to form such conductors as the digitlines or wordlines used in memory devices (see commonly-owned U.S. Pat. No. 5,492,597, hereby incorporated herein by reference). Other metallic silicides used in gate stacks include cobalt silicide (xe2x80x9cCoSixxe2x80x9d), molybdenum silicide (xe2x80x9cMoSixxe2x80x9d), and titanium silicide (xe2x80x9cTiSixxe2x80x9d). These metallic silicides have lower resistivity and are easier to fabricate than other conductors used for this purpose. However, metallic silicides are prone to oxidization. Furthermore, the metal components of the metallic silicides react chemically when they contact other elements. These properties present several problems, including degradation of the semiconductor element and peeling of the metallic silicide film. To compensate for these problems, a polysilicon layer is usually disposed between a gate dielectric layer and the metallic silicide film, and a dielectric cap layer is usually disposed above the metallic silicide film to isolate the metallic silicide.
FIGS. 14-19 illustrate, in cross section, a conventional method of forming a gate stack having a metallic silicide film layer. FIG. 14 illustrates a gate dielectric layer 204 such as silicon dioxide (SiO2) grown (by oxidation) or deposited (by any known industry standard technique, such chemical vapor deposition or the like) on a silicon substrate 202. A polysilicon layer 206 is formed on top of the gate dielectric layer 204, as shown in FIG. 15. The polysilicon layer 206 is then subjected to an ion implantation with gate impurities (not shown). As shown in FIG. 16, a metallic silicide film 208 is deposited on the polysilicon layer 206. The structure is then subjected to a heat treatment for about 30 minutes at a temperature between about 850xc2x0 C. and 950xc2x0 C. for activation of the impurities in the polysilicon layer 206 and to anneal the metallic silicide film 208. The heat treatment temperature level is dictated by the temperature required to anneal the metallic silicide film 208. The annealing of the metallic silicide film 208 is used to reduce its resistivity.
As shown in FIG. 17, a silicon dioxide cap 210 is then deposited on the metallic silicide film 208 at temperatures over 600xc2x0 C. by chemical vapor deposition (xe2x80x9cCVDxe2x80x9d), low pressure chemical vapor deposition (xe2x80x9cLPCVDxe2x80x9d), or the like. A resist 212 is then formed and patterned on the silicon dioxide cap 210, as illustrated in FIG. 18. The layered structure is then etched and the resist 212 is stripped to form a gate stack 214, as illustrated in FIG. 19. However, this etching results in pitting on the gate dielectric layer 204. This pitting is illustrated in FIG. 20 wherein a plurality of pits 216 is distributed on the gate dielectric layer 204 between the gate stacks 214.
This pitting is also illustrated in FIG. 19. A pit in the dielectric layer 204 may be shallow, such as shallow pit 218. However, a deep pit, such as deep pit 220, can extend through the gate dielectric layer 204 and into the silicon substrate 202. The pitting into the silicon substrate 202 will cause junction leakage, refresh problems, and potential destruction of the component. At present, most gate dielectric layers are about 80 xc3x85 thick. However, as semiconductor devices continue to be miniaturized, these gate dielectric layers will become thinner. As the gate dielectric layers become thinner, it is more likely that pitting will penetrate through the gate dielectric layer to contact the silicon substrate and cause the aforementioned problems.
Therefore, it would be advantageous to develop a technique which minimizes or eliminates pitting on the gate dielectric layer caused by gate stack etching, while using state-of-the-art semiconductor device fabrication techniques employing known equipment, process steps, and materials.
The present invention relates to the reduction of the temperature during the fabrication of the gate stack to eliminate the formation of silicon clusters within a metallic silicide film of the gate stack. The elimination of the formation of the silicon clusters minimizes or eliminates damage to the gate dielectric layer and/or silicon substrate during the gate stack formation. The present invention also includes methods for implanting the gate stack layers to disperse the silicon clusters (if they are present in the metallic silicide film) prior to the gate etch step.
One aspect of the method of the present invention begins by forming a gate dielectric layer on a silicon substrate. A polysilicon or amorphous silicon layer (hereinafter xe2x80x9cpolysilicon layerxe2x80x9d) is then formed on top of the gate dielectric layer. The polysilicon layer is subjected to an ion implantation with gate impurities and a non-annealed metallic silicide film is thereafter deposited atop the polysilicon layer. A dielectric cap layer is then deposited over the metallic silicide film at a sufficiently low temperature such that the metallic silicide does not anneal. A resist mask is placed over the cap layer and the structure is etched down to the gate dielectric layer to form a gate stack.
Metallic silicides are generally represented by the formula xe2x80x9cMSixxe2x80x9d wherein: xe2x80x9cMxe2x80x9d is the metal component (i.e., cobalt xe2x80x9cCo,xe2x80x9d molybdenum xe2x80x9cMo,xe2x80x9d titanium xe2x80x9cTi,xe2x80x9d tungsten xe2x80x9cW,xe2x80x9d and the like), xe2x80x9cSixe2x80x9d is silicon, and xe2x80x9cxxe2x80x9d is the number of silicon molecules per metal component molecule (xe2x80x9cxxe2x80x9d is usually between about 2 and 3). Metallic silicide films tend to peel when a low ratio of silicon to metal component is used for gate stack formation (e.g., when xe2x80x9cxxe2x80x9d is less than 2). In order to reduce the stress of metallic silicide film which causes peeling, a silicon rich metallic silicide film is used in gate stack formation. In particular with the use of WSix, an xe2x80x9cxxe2x80x9d of about 2.3 is preferred.
In prior art techniques, the metallic silicide is annealed to form a crystalline structured metallic silicide film 502, as illustrated in FIG. 23, between a polysilicon layer 504 (atop a gate dielectric layer 506, which is on a silicon substrate 508) and a silicon dioxide layer 510 (below a dielectric cap 512). However, when a silicon rich metallic silicide is used, the annealing step causes the silicon within the metallic silicide to form clusters 514 inside the crystalline structured metallic silicide film 502. These silicon clusters 514 can also form during the subsequent high temperature steps, even if the annealing step does not take place. In specific process terms, the step of forming a dielectric cap over the metallic silicide can exceed 600xc2x0 C., particularly when deposition techniques such as LPCVD and sputtering are used. These high temperature steps can cause the formation of silicon clusters 514 within the crystalline structured metallic silicide film 502. This can be seen in FIG. 21 wherein a large plurality of pits 304 is formed in the surface of the gate dielectric layer 306 between a plurality of gate stacks 302 (high temperature cap formation only, no annealing step).
It has been found that the pitting on the gate dielectric layer during the full gate stack (cap/metallic silicide/polysilicon) etch is caused by the presence of the silicon clusters inside the metallic silicide film. The etch rate of these silicon clusters has been found to be about 1.2 times that of the metallic silicide film (in the case of tungsten silicide film) during the gate stack etch. Thus, the etch tunnels into the metallic silicide at each silicon cluster. This tunneling is, in turn, translated into the surface of the gate dielectric layer, thereby forming the pits.
By preventing the growth and formation of the silicon clusters in the metallic silicide film, the problem of pitting on the silicon substrate during the gate stack etch can be eliminated. Although prior art techniques anneal the metallic silicide film to reduce its resistivity and consequentially forming the undesirable silicon clusters, it has been found that, for most purposes, the metallic silicide film has sufficiently low resistivity without annealing. Thus, one aspect of the method of the present invention eliminates annealing the metallic silicide film. Although the step of annealing the metallic silicide film also activates gate impurities, the activation of the gate impurities can be completed during subsequent heat cycles after the etching of the gate stack, such as during shallow junction formation.
In a preferred variation of the method, the dielectric cap is selectively deposited on an upper surface of the metallic silicide film at low temperatures. The dielectric cap material is preferably silicon nitride. The deposition of the silicon nitride layer is carried out at between about 400 and 600xc2x0 C., which temperature does not anneal the metallic silicide film, and thus does not result in the growth and formation of silicon clusters in the metallic silicide film. It is, of course, understood that the cap can include silicon dioxide layers, or the like, so long as deposition is performed at temperatures below about 600xc2x0 C. Forming the cap by selectively depositing silicon nitride by plasma-enhanced chemical vapor deposition (xe2x80x9cPECVDxe2x80x9d) is also preferred, since only one surface of the substrate is covered by the dielectric material which eliminates the necessity of removing the cap material from the semiconductor substrate back surface, thus providing a process cost advantage.
FIG. 24 is a side cross-sectional view of a layered gate stack structure of the present invention prior to etching, depicting a silicon nitride cap 602, a silicon dioxide layer 604, a metallic silicide film 606, a polysilicon layer 608, a gate dielectric layer 610, and a silicon substrate 612. Since no high temperature cycle occurs during the layered gate stack structure formation, the metallic silicide film 606 does not form a crystalline structure, nor does it contain silicon clusters. Thus, as illustrated in FIG. 22, the method of the present invention does not initiate damage or pitting on the gate dielectric layer 402 during the etching and formation of the gate stacks 404.
In situations where a high temperature heat cycle (cap deposition and/or annealing) is required, an ion implantation into the metallic silicide film can be performed to amorphize the metallic silicide film (i.e., disperse the silicon clusters back into the metallic silicide film) before masking and etching. The implantation ions can be silicon, tungsten, argon, or the like, or a dopant (phosphorous, arsenic, boron, and the like). The implantation can be performed before and/or after the cap deposition. The implantation energy is preferably between about 20 keV and 200 keV. The ion dose ranges from between about 1E13 and 1E16. The implantation energy and dose depend on the metallic silicide film thickness, the metallic silicide composition (i.e., ratio of silicon to metal component), the anneal heat cycle temperature, and the implantation ion used. However, it is preferred that the peak of the implantation occur at about the middle of the metallic silicide film. Furthermore, it is preferred that the dopant ion (phosphorous, arsenic, boron, and the like) amorphize the metallic silicide film. For example, for a metallic silicide film which is about 1800 xc3x85 thick and annealed at about 850xc2x0 C. for about 30 minutes, a phosphorous implantation at about 75 keV and 1E15 is required to amorphize the metallic silicide.
It is, of course, understood that if a lower resistivity in the metallic silicide is required for a specific application, the gate stack can be subjected to a heat cycle after gate stack etching to anneal the metallic silicide in the gate stack. However, if the gate stack is annealed after formation, the anneal temperature must be increased by about 30xc2x0 C. to 50xc2x0 C. to achieve the same resistivity.